He²⁺ dynamics and ion cyclotron waves in the downstream of quasi‐perpendicular shocks: 2‐D hybrid simulations

Abstract

The free energy provided by the ion temperature anisotropy is considered to be the source of ion cyclotron waves in the downstream of a quasi-perpendicular shock. Besides the proton cyclotron waves excited by the proton temperature anisotropy, He 2 + is decelerated differentially from the protons by the shock due to its different charge-to-mass ratio and forms a bunched ring-like distribution in the immediate downstream of the quasi-perpendicular shock. However, how the helium cyclotron waves associated with the anisotropic distribution of He 2 + are excited is still in debate. In this paper, with two-dimensional (2-D) hybrid simulations, we investigate He 2 + dynamics and its role in the ion cyclotron waves downstream of quasi-perpendicular shocks (the proton plasma beta in the upstream is 0.4). A bunched ring-like distribution of He 2 + is formed in the immediate downstream of the quasi-perpendicular shocks; then it evolves into a shell-like distribution. At last, a bi-Maxwellian distribution of He 2 + is generated in the far downstream. In the medium and low Mach number shocks, besides the proton cyclotron waves excited near the shock front, there is another enhancement of the magnetic fluctuations in the downstream. The results show that the helium cyclotron waves can be driven directly by the bunched ring-like distribution of He 2 + in a low or medium Mach number quasi-perpendicular shock. The relevance of our simulation results to the satellite observations is also discussed in this paper.

Stable ring beam of solar wind He2+ in the magnetosheath

“…All the velocity components became Maxwellian, where the perpendicular heating (v x and v y ) is dominant. The latter case (Run 1C) was consistent with the previous simulations [e.g., Glassmeier, 1993, 1994;Lu and Wang, 2005;Hao et al, 2014]. Figure 5b).…”

“…It should also be noted that parallel diffusion must be too strong in the present cases because the cross‐field diffusion is artificially suppressed in both 1‐D and 2‐D simulations [ Jokipii et al , ]. Contrary to Hao et al [], the helium cyclotron waves must provide a minor contribution to the scattering of the beam, because of the low‐density ratio of He 2+ /proton, as well as the low Mach number regime (in Runs 1A or 2A/2B), where the intensity of induced helium waves might be insufficient to scatter the He 2+ ions.…”

“…All the runs indicated large fluctuations immediately behind the shock, which in part constitute proton cyclotron waves driven by the proton temperature anisotropy T ⊥ / T ∥ >1 in the quasi‐perpendicular shock downstream [e.g., Gary et al , ]. It has obviously been confirmed that there were larger fluctuations in both components for the case of larger He 2+ density, which should be the contribution of the helium cyclotron waves driven by the ring beam distribution of He 2+ [ Lu and Wang , ; Hao et al , ]. In Run 1A, while was rapidly decaying to the background level within 10 L p from the shock, the reduction in to the background took longer, approximately 50 L p .…”

Turbulence‐Driven Magnetic Reconnection in the Magnetosheath Downstream of a Quasi‐Parallel Shock: A Three‐Dimensional Global Hybrid Simulation

“…The magnetosheath, which is full of turbulent plasmas, is formed behind the bow shock after the high‐speed solar wind interacts with the Earth's magnetosphere. When the shock is quasi‐perpendicular (the angle between the shock normal direction and upstream magnetic field θ Bn > 45°), the shocked magnetosheath is dominated by ion cyclotron waves and mirror waves that are excited by an ion temperature anisotropy (e.g., Hao et al, ; Lee et al, ; Lu & Wang, ; McKean et al, ; Winske & Quest, ). However, the characteristics of the magnetosheath behind the quasi‐parallel shock ( θ Bn < 45°) is still unclear.…”

Formation of downstream high‐speed jets by a rippled nonstationary quasi‐parallel shock: 2‐D hybrid simulations

“…After only one bounce (full gyromotion), these reflected ions gain enough energy to pass through the shock front and penetrate into the downstream region [Sckopke et al, 1983;Lembège, 1990;Hada et al, 2003;Mazelle et al, 2003;Lembège et al, 2004;Ofman et al, 2009;Yang et al, 2009aYang et al, , 2009bYang et al, , 2012Ofman and Gedalin, 2013]. Such a process results in anisotropic distributions of the downstream ions and leads to the excitation of Alfven ion cyclotron and mirror waves [Gary et al, 1976[Gary et al, , 1994Winske and Quest, 1988;Gary, 1992;McKean et al, 1995aMcKean et al, , 1995bMcKean et al, , 1996Wang, 2005, 2006;Shoji et al, 2009;Hao et al, 2014]. In a quasi-parallel shock, the reflected ions do not have a trajectory restricted to one gyromotion upstream but can escape upstream along the magnetic field.…”